This invention relates to multipoint strip chart recorders for recording trend lines for the values of the input signals representing a number of points and particularly to recorders designed for industrial use where slow chart speeds are desirable and where it is also desirable to be able to view with ease the most recent part of the record. Industrial strip chart recorders typically have a horizontally oriented paper drive roll which is motor driven so that chart paper is fed at a predetermined rate from a supply system, such as a paper supply roll or supply tray in the back of the recorder case, over the chart drive roll and down the front of the case, frequently over a vertical table, to a collection system, such as a take-up roll or a paper collection tray, depending on whether the chart is in the form of a roll or a fan-fold. In industrial applications the chart is frequently fed at a speed of 1 or 2 cm/hr with the intent that the position of any record on the chart should be indicative not only of the value of the variable being recorded but also of the time at which the variable was measured.
In single point recording the record is typically made by a pen which continuously draws a line on the chart indicating the trend of the variable. In such recorders the pen is frequently constructed so that it is recording the current value of the variable at a point on the front half of the drive roll so that the current value can be easily viewed from in front of the recorder. Such pen structures utilize a long curved end so that the mark made by the end of the pen is not obscured either by the scale of the recorder, typically just above the feed roller and toward the front, or by the pen carriage and its supporting track, usually just above the paper drive roll.
In multipoint recording when slow chart speeds are used the problem of getting good visibility of the most recent record is complicated by the size of the required printing mechanism. Generally these recorders are designed to make their record on the top portion of the paper drive roll, preferably at top-dead-center, in order to accommodate the large printing mechanism. That position, in combination with the presence of a scale above the chart, almost completely obscures the most recent record. Others have attempted to mount the printing mechanism forward of top-dead-center, but these arrangements have been found to be unsatisfactory because the printing mechanism, by itself, substantially obscures the recent record when it is mounted in a forward position.
In recorders using slow chart speeds, the problem of reading the most recent record is at its worst. In fact, that part of the record may not be visible to the operator until more than an hour after the record is made. Such a time delay is frequently found to be intolerable. Also, since recorders of this type are frequently arrayed in large numbers on adjacent vertical panels, they must not require that a great effort be expended in monitoring the recent values and their trend, for otherwise mistakes resulting from erroneous monitoring will be the likely result. In order for the user to not only be able to read the most recent record but also to have confidence in the accuracy of his efforts he should be able to see some blank chart above the most recent recorded values.
It is an object of this invention to provide a method and apparatus for effecting a multipoint record on a strip chart recorder while at the same time providing good visibility for the most recent portion of that record.